1. Field of the Invention
The invention relates to ferroelectric transistors which are suitable for integrating an integrated circuit to a higher degree and operating an integrated circuit with a greater speed and to a manufacturing method thereof.
2. Description of the Prior Art
Research and Development of semiconductor storage devices which are non-volatile and implemented with ferroelectric material have been actively pursued. For example, a device called an MFSFET has been proposed. This FET (Field Effect Transistor) is constructed in an MFS (Metal/Ferroelectric/Semiconductor) structure wherein a semiconductor, a ferroelectric and a metal are sequentially deposited. This device has a simple structure so that it can be readily reduced in its size. Additionally, data stored in this device may be read out in a speedy and undestructive manner. It is, however, difficult to appropriately control the boundary (or interface) between the ferroelectric and the semiconductor of this device in manufacturing it. In other words, a high quality boundary is difficult to accomplish. Thus, so far, this type of device has not yet been brought into market.
An FCG (Ferroelectric-Capacitor and transistor-Gate connection ) device has also been proposed. For more details, see "1996 Symposium on VLSI Technology Digest of Technical Papers," pp. 56 to 57, 1996. This publication is hereby incorporated by reference. An FCG device is constructed in such a manner that each of a ferroelectric capacitor and a MOSFET (Metal/Oxide/Semiconductor FET)is provided in a separate location on an underlying layer or supporting layer and that the ferroelectric capacitor and the gate electrode of the MOSFET are locally wired. Thus, since a ferromagnetic portion is not formed on the semiconductor, the above-mentioned quality problem of the interface does not occur. Further, the ratio between the electric capacitance of the ferroelectric capacitor and gate capacity can be relatively freely set up by changing the size of the ferroelectric film or the gate isolating film. Accordingly, it is easy to apply a voltage to the ferroelectric capacitor so as to inverse the polarization of the ferroelectric. However, its reduction in size is not easy because of the complexity in structure as opposed to the case of an MFSFET.
As discussed above, it is difficult to appropriately control the interface between the ferroelectric and the semiconductor of an MFSFET in its manufacture. On the other hand, an FCG device has a disadvantage that it can not readily be reduced in its size. In order to overcome these problems, an MFMISFET has been proposed. An MFMISFET is constructed in such a manner that an MFM (Metal/Ferroelectric/Metal) structure, which comprises a ferroelectric film is sandwiched by a pair of metal films from both sides, is provided on the gate electrode of a MOSFET. Thus, the gate portion of this device has an MFMIS (Metal/Ferroelectric/Metal/Insulator/Semiconductor) structure. Since this structure has a ferroelectric film formed on a metallic film, a good interface can be achieved. Reduction in its size is also easily done. However, this MFMISFET has following disadvantages:
(1) A ferroelectric film has to be formed by means of a heated process at such a high temperature of approximately 800.degree. C. This results in an outer diffusion of the elements constituting the ferroelectric so that channel regions are contaminated, thereby degrading the characteristics of the MOSFET device.
(2) A wiring or an interconnection layer is formed above the MFMISFET, which necessitates an interlayer insulation film or an interlayer dielectric film on the MFMISFET in order to electrically isolate the wiring layer from the underlying conductor. Thus, if a thermal distortion occurs at the interlayer insulation layer, a relatively large stress is generated onto the ferroelectric film, which degrades the characteristics of the ferroelectric. As a (thermal) stress is also generated in the course of a thermal or heating process (or heat treatment), the number of thermal processes should preferably be kept to as minimum as possible once the ferroelectric film is formed.
(3) A heated process in a hydrogen gas is sometimes necessary so as to restore the damage on the MOSFET (for example, crystal defects caused by an ion implantation). In such an instance, a reductive reaction of the ferroelectric film sometimes occurs, thereby degrading the characteristics of the ferroelectric material.
The above-described problems are due to the thermal processes.
Accordingly, objects of the present invention are to provide a ferroelectric transistor having a characteristic which is not degrated in the course of a thermal process, and the manufacturing method of the transistor.